cannot echo rostopics published by gazebo

edit

I tried to attach a bumper on moveit setup assistant generated panda arm urdf

arm.xacro:

<link name ="bumper_link">
    <collision>
      <geometry>
        <box size="0.01 0.02 0.01"/>
      </geometry>
    </collision>
   <visual>
        <geometry>
          <box size="0.01 0.02 0.01"/>
        </geometry>
   </visual>
  <inertial>
        <mass value="0.1" />
         <inertia ixx="0.03" iyy="0.03" izz="0.03" ixy="0.0" ixz="0.0" iyz="0.0" />
      </inertial>
</link>
<joint name="bumper" type ="revolute">
<axis xyz="0 1 0" />
   <limit effort="12" lower="-4.8973" upper="1.8973" velocity="2.6100" />
    <origin xyz="0 0 0.06" rpy="0 0 0"/>
    <parent link="panda_leftfinger"/>
    <child link="bumper_link"/>
  </joint>

arm.gazebo:

<?xml version="1.0"?>
<robot>
<gazebo>
        <plugin name="gazebo_ros_control" filename="libgazebo_ros_control.so">
            <robotNamespace>/</robotNamespace>
        </plugin>
    </gazebo>
 <gazebo reference="bumper_link">
  <sensor name="bumper" type="contact">
    <always_on>1</always_on>
    <update_rate>30</update_rate>
    <contact>
    <collision>bumper_link_collision</collision>
    </contact>
   <plugin name="gazebo_ros_bumper_controller" filename="libgazebo_ros_bumper.so">
        <bumperTopicName>bumper_topic</bumperTopicName>
   <always_on>true</always_on>
      <updateRate>30.0</updateRate>
        <frameName>bumper</frameName>
    </plugin>
   </sensor>
</gazebo>
</robot>

launch file:

<?xml version="1.0" encoding="UTF-8"?>
<launch>

  <arg name="world" default="empty"/> 
  <arg name="paused" default="true"/>
  <arg name="use_sim_time" default="false"/>
  <arg name="recording" default="false"/>
  <arg name="debug" default="true"/>
  <arg name="gui" default="true"/>

  <include file="$(find gazebo_ros)/launch/empty_world.launch">
    <arg name="paused" value="$(arg paused)"/>
    <arg name="use_sim_time" value="$(arg use_sim_time)"/>
    <arg name="recording" value="$(arg recording)"/>
    <arg name="gui" value="$(arg gui)"/>
    <arg name="debug" value="$(arg debug)"/>
  </include>
  <param name="robot_description" command="$(find xacro)/xacro --inorder $(find robot_description)/urdf/arm.xacro" />
  <node name="arm_spawn" pkg="gazebo_ros" type="spawn_model" output="screen"
   args="-urdf -param robot_description -model panda -x 0 -y 0 -z 1" />
</launch>

rostopic list gives:

    /bumper_topic
    /clock
    /gazebo/link_states
    /gazebo/model_states
    /gazebo/parameter_descriptions
    /gazebo/parameter_updates
    /gazebo/set_link_state
    /gazebo/set_model_state
    /rosout

but rostopic echo gives no output for /bumper_topic, /gazebo/....etc

Could someone help with this? Thanks!